The present invention relates to an optical disc recording and/or playback apparatus and method, for playback of information signal recorded in an optical disc having a plurality of recording layers or data layers, and also to a focus servo controller and focus servo pull-in method, for controlling the focusing of a light beam to any one of a plurality of recording layers.
Conventionally, as a recording medium for a variety of information such as audio information, video information, etc., optical discs are widely used from which information recorded therein is read out optically, namely, using a light beam. There have recently been proposed such optical discs having formed therein many recording layers for the purpose of recording a further increased amount of information.
An optical disc having a plurality of recording layers formed therein is known from the disclosure in the Japanese Unexamined Patent Publication No. 8-235641.
Referring now to FIG. 1, there is illustrated in the form of a sectional view a conventional optical disc, that is, the optical disc having a plurality of recording layers as in the above Japanese Unexamined Patent Publication No. 8-235641. As shown in FIG. 1, the optical disc, generally indicated with a reference 1, includes a first record carrier 4a having a first recording layer 3a formed on one side of a first substrate 2a which is light-transmissive and a second record carrier 4b having a second recording layer 3b formed on one side of a second substrate 2b which is also light-transmissive. The first and second record carriers 4a and 4b are joined to each other with the sides of the first and second recording layers 3a and 3b having formed thereon the first and second recording layers 3a and 3b, respectively, being opposite to each other as well as with a light-transmissive adhesive layer 5 being disposed between the first and second record carries 4a and 4b. 
Information such as video information or the like is recorded as pit patterns 6a and 6b each defined by tiny pits and lands on the sides of the first and second substrates 2a and 2b forming the first and second record carriers 4a and 4b, respectively, on which the first and second recording layers 3a and 3b are formed, respectively. The first recording layer 3a is formed on the pit pattern 6a formed on the first record carrier 4a by forming a translucent or semi-reflective layer of SiN, SiO2 or the like along the pit pattern 6a by evaporation, sputtering or the like. The first recording layer 3a reflects 20 to 50%, and transmits 30 to 80%, of a light beam incident upon it. The second recording layer 3b is formed on the pit pattern 6b formed on the second record carrier 4b by forming an aluminum evaporated layer or the like along the pit pattern 6b. The second recording layer 3b reflects 60% or more of an incident light, but it will reflect 20 to 50% of a light beam having passed through the first recording layer 3a and reflected from the second recording layer 3b. 
From the optical disc 1 having the first recording layer 3a which is a translucent or semi-reflective as in the above, information recorded in the second recording layer 3b can be read by directing a light beam L2 in the same direction as that in which a light beam L1 is directed towards the first recording layer 3a as shown in FIG. 1. In this case, either information recorded in the first recording layer 3a or information recorded in the second recording layer 3b is selected for reading by changing the focused position of the light beams L1 and L2 directed towards the optical disc 1.
Also, as the optical disc having a plurality of recording layers, there has been proposed a one shown in FIG. 2. This example of optical disc is generally indicated with a reference 11. As shown in FIG. 2, the optical disc 11 includes a first record carrier 14a having a first recording layer 13a formed on one side of a first substrate 12a which is light-transmissive and a second record carrier 14b having a second recording layer 13b formed on one side of a second substrate 12b which is also light-transmissive. The first and second record carriers 14a and 14b are joined to each other with an adhesive layer 15 of a light-transmissive, ultraviolet-settable resin or the like. They are joined in parallel with each other so that the second substrate 12b is disposed above the first recording layer 13a. On the second recording layer 13b, there is provided a protective layer 17 for the second recording layer 13b. 
Information such as audio information or the like is recorded as pit patterns 16a and 16b each defined by tiny pits and lands on the sides of the first and second substrates 12a and 12b forming the first and second record carriers 14a and 14b, respectively, on which the first and second recording layers 13a and 13b are formed, respectively. The first recording layer 13a transmits only a light beam having a predetermined wavelength while reflecting a light beam having any other wavelength. It is formed on and along the pit pattern 16a formed on the first record carrier 14a. The first recording layer 13a is formed to have a multilayered structure consisting of five layers of Si3N4, SiO2, Si3N4, SiO2 and Si3N4, respectively, for example. The first recording layer 13a reflects approximately 34% of a light beam of 635 nm in wavelength while reflecting little of a light beam of 780 nm in wavelength.
On the pit pattern 16b formed on the second record carrier 14b, there is formed the second recording layer 13b by forming a highly reflective aluminum evaporated layer or the like along the pit pattern 16b. The second recording layer 13b reflects 80% or more of a light beam having passed through the first recording layer 13a. It reflects 84% or more of a light beam of 780 nm in wavelength, most of which is transmitted through the first recording layer 13a, while reflecting about 38% of a light beam of 635 nm in wavelength, of which about 34% is reflected by the first recording layer 13a. 
In case of the optical disc 11 constructed as in the above, a light beam is directed from the side of the first substrate 12a of the first record carrier 14a to read information recorded in the first and second recording layers 13a and 13b. At this time, a light beam L3 of 780 nm in wavelength, used to read the information, will be transmitted through the first recording layer 13a and incident upon the second recording layer 13b, and a portion of the light beam will be reflected as a return light from the second recording layer 13b. By detecting the return light, information such as the audio information or the like recorded in the second recording layer 13b is read. Since the light beam L3 of 780 nm in wavelength is used to play back a so-called compact disc which is a read-only optical disc of 12 cm in diameter, information recorded in the second recording layer 13b can be read by a general-purpose or versatile optical disc player.
Also, a light beam L4 of 635 nm in wavelength, directed from the side of the first substrate 12a of the first record carrier 14a, will be reflected from both the first and second recording layers 13a and 13b to provide a return light each. More specifically, since the first reflecting layer 13a reflects about 34% of a light beam of 635 nm in wavelength, the light beam will pass through the first recording layer 13a and incident upon the second recording layer 13b to provide a return light from the second recording layer 13b. The second recording layer 13b reflects approximately 34% of the light beam incident upon it.
By focusing the light beam L4 of 635 nm in wavelength on either the first or second recording layer 13a or 13b of the optical disc 11, information recorded in either the first or second recording layer 13a or 13b can be read.
In case of the optical disc 1 in which the first and second record carriers 4a and 4b are joined to each other with the sides thereof on which the first and second recording layers 3a and 3b are formed, respectively, being opposite to each other as shown in FIG. 1, information recorded in either the first or second recording layer 3a or 3b can selectively be read by focusing a light beam on either the first or second recording layer 3a or 3b of the optical disc 1. Since in the optical disc 1, the first and second recording layers 3a and 3b are disposed close to each other, an error will possibly take place in detecting the focused position of a light beam to detect the first or second recording layer 3a or 3b. It will be difficult to positively focus the light beam on the first or second recording layer 3a or 3b and thus it is not possible to accurately read any desired information from the optical disc 1.
Also it has been proposed to select either the first or second recording layer 3a or 3b by detecting a difference in reflectance between the first and second recording layers 3a and 3b to detect on which of the first and second recording layers 3a and 3b a light beam incident upon the optical disc 1 is focused. Also in this case, since the first and second recording layers 3a and 3b has the nearly same reflectance for the light beam incident upon the optical disc 1, the difference in reflectance for the light beam cannot be used to select either the first or second recording layer 3a or 3b. 
In case of the optical disc 11 having the first recording layer 13a which transmits only a light beam having a predetermined wavelength while reflecting a light beam having any other wavelength, as shown in FIG. 2, information recorded in the first recording layer 13a can be read by the versatile optical disc player in which a compact disc is to be used. To read information recorded in the first and second recording layers 13a and 13b, a dedicated optical disc player using a light beam of 635 nm in wavelength is necessary. Since the first and second recording layers 13a and 13b have the nearly same reflectance for the light beam incident upon the optical disc 11, either the first or second recording layer 13a or 13b cannot be selected by detecting the difference in reflectance of the recording layers for the light beam.
As in the above, although the optical discs having so far been proposed have formed therein a plurality of recording layers for recording an increased amount of information, desired information cannot be read easily and accurately because it is difficult to focus a light beam accurately on a selected one of a plurality of recording layers.
Accordingly, it is an object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing an optical recording medium recorder and/or player for playback of information recorded in an optical recording medium having formed therein a plurality of recording layers intended for an increased recording capacity, adapted to easily select each of the recording layers and positively read information recorded in a desired recording layer.
It is another object of the present invention to provide a focus servo pull-in method and unit enabling to focus a light beam on a desired one of a plurality of recording layers formed in an optical recording medium and positively read information recorded in the recording layer on which the light beam is focused.
It is still another object of the present invention to provide an optical recording medium playback method for selectively reading, with a light beam, a plurality of recording layers formed in the optical recording medium.
The above object can be attained by providing an optical disc recorder and/or player including according to the present invention:
an optical pickup having a photodetector to direct a light beam towards any one of a plurality of recording layers formed one over the other in the optical disc and detect a return portion of the light beam reflected back from the recording layer; and
an identifier to compare output signal from the photodetector with a predetermined reference value and identify, based on the comparison result, which of the plurality of recording layers in the optical disc has focused thereon the light beam from the optical pickup.
Also the above object can be attained by providing an optical disc recorder and/or player including according to the present invention:
an optical pickup including:
a light source to emit a light beam for incidence upon any one of a plurality of recording layers formed one over the other in the optical disc and different in reflectance from each other;
an objective lens to converge the light beam emitted from the light source to any one of the plurality of recording layers; and
a photodetector including a plurality of detector blocks to detect a return portion of the light beam reflected back from the recording layer;
a signal generator to generate, from output signals from the detector blocks of the photodetector, a focus error signal and a signal indicative of a sum of the output signals from the detector blocks; and
a controller including a servo control block to move the objective lens optical-axially thereof based on the focus error signal from the signal generator; and an identifier to identify, based the focus error signal and sum signal from the signal generator, which of the plurality of recording layers in the optical disc has focused thereon the light beam from the optical pickup.
In the above optical disc recorder and/or player, the identifier identifies which of the plurality of recording layers in the optical disc has focused thereon the light beam from the optical pickup, depending on whether when the focus error signal from the signal generator is at a predetermined signal level, the level of the sum signal from the signal generator is below a threshold for any of the plurality of recording layers formed in the optical disc. Also, the identifier identifies which of the plurality of recording layers in the optical disc has focused thereon the light beam from the optical pickup, depending on whether when the focus error signal from the signal generator crosses the signal zero level, the level of the sum signal from the signal generator is below the threshold for any of the plurality of recording layers formed in the optical disc.
Also the above object can be attained by providing a focus servo controller including according to the present invention:
a servo control block for servo control to move an objective lens optical-axially thereof based on a focus error signal generated from an output signal from each of detector blocks of a photo detector which detects a return portion of a light beam reflected from any of a plurality of recording layers formed one over the other in an optical disc and different in reflectance from each other; and
an identifier to identify, based on the focus error signal and the sum of output signals from the detector blocks, which of the plurality of recording layers in the optical disc has focused thereon the light beam from an optical pickup.
Also the above object can be attained by providing a focus servo pull-in method including, according to the present invention, steps of:
generating a focus error signal and a sun signal from output signals from detector blocks of a photodetector to detect a reflected return portion of a light beam incident through an objective lens upon any of a plurality of recording layers formed one over the other in an optical disc and different in reflectance from each other;
identifying, based on the focus error signal and sum signal, which of the plurality of recording layers in the optical disc has focused thereon the light beam from an optical pickup; and
closing a servo loop to move the objective lens optical-axially thereof based on the focus error signal.
These objects and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.